Technical Field
The invention concerns a method of operating a wind power installation and a wind power installation for carrying out the method.
Description of the Related Art
As state of the art attention is directed in particular to ‘Grid Integration of Wind Energy Conversion Systems’, Siegfried Heier, 1998, therein in particular pages 263 and so forth as well as U.S. Pat. No. 7,345,373 and WO 01/86143. The most relevant state of the art is document WO 01/86143.
That document discloses the teaching of reducing the power of a wind power installation when the network frequency, that is to say the frequency of the network into which the wind power installation feeds its electrical power, exceeds a given value above the target frequency.
In the case of Central European networks the target frequency is usually at 50 Hz whereas in the case of US networks it is at 60 Hz. The network is also referred to as the grid or power grid.
At the same time however there are also sometimes slight network frequency fluctuations which are dependent on how greatly the ratio of the power produced by the energy producers connected to the electrical network is balanced out in relation to the power taken by the consumers, that is to say those who are connected to the electrical network and take electrical energy in order therewith to operate any electrical equipment.
If, for example, the power supply from the generators is above that which the consumers connected to the network are taking in terms of power, the network frequency rises. Conversely the frequency can also fall below the target frequency, for example below 50 Hz, if the power supply offered is less than that which is being taken by the electrical consumers connected to the network.
Network management, that is to say the management of producers and also large consumers, is usually implemented by the network operators. In that case network management can provide quite different regulating mechanisms, for example for automatically switching on certain generators (for example gas fired power stations), automatic switch-off of given large consumers or also the use of pumped storage plants and the like. In normal operation even the network management of large supply networks constantly succeeds in keeping the network frequency in the region of the target frequency, in which respect minor deviations are certainly allowed. Those minor deviations however should generally not exceed the region of +0.1%. It will be appreciated that the network management can also involve switching on further networks which are connected to the network in order thereby to feed additional power into the network or to take it from the network and feed it into other networks.
For wind power installations, document WO 01/086143—as already stated above—already teaches reducing the power below the currently available power if a given network frequency value is exceeded, for example a value which is 3% above the target value of the network frequency (for example over 50 Hz). The document, WO01/086143, further teaches that, if the frequency continues to rise, the power is linearly further reduced, in dependence on the further rise in the network frequency.